This invention relates generally to level measurement, and more particularly to an improved inclination gauge providing both a digital and an analog read-out.
As used in mechanics, level is a term referring to any direction that is at right angles to the force of gravity. Because of the earth's curvature, this direction is not precisely horizontal, but the deviation from the horizontal is negligible for short distances. The tool kit of any competent bricklayer or carpenter invariably includes a spirit level. However, the use of such levels is by no means limited to the field of construction, for this level is an essential component of many delicate physical, astronomical and engineering instruments.
Operation of the spirit level depends on the simple principle that an air bubble seeks the highest point in a container enclosing the liquid in which the bubble is formed. In a standard spirit level, the glass tube housing the liquid is either slightly curved with its convex side upward, or the tube is ground with a curved inner core.
The glass tube is supported on a rigid base or bar, and the bubble therein always comes to equilibrium at the same point whenever the bar has the same slope with respect to the horizontal plane. By providing the spirit level tube with a graduated scale, the level may be made a sensitive gauge for measuring changes in the angle of inclination.
From the practical standpoint, the value of a spirit level in providing an accurate indication of inclination depends on the ability of the user to discern the precise position of the air bubble with respect to the scale indicia on the tube. While the condition of the user's eyesight is a factor in this regard, as well as the angle at which he views the bubble, for many ordinary level applications, the fact that the readings taken by the user are somewhat inexact may not be a serious drawback; but in other situations, even a slight error cannot be tolerated.
But whether or not the user of the spirit level is capable of reading the level without difficulty, the fact is that the user, in order to obtain a correct reading, must exercise care, and this consumes time--which, in a procedure requiring frequent level readings, adds materially to the cost of the operation.
While the invention will be described in connection with an inclination gauge which supplants or supplements conventional spirit levels of the type used in the construction industry, it is to be understood that the gauge in accordance with the invention is also useful as a surveyor's level attachable to a telescope to determine differences in elevation, and in other instruments which entail the measurement of inclination.
In applicants' above-identified copending application, there is disclosed an inclination gauge having a digital readout whereby the observer, regardless of the condition of his eyesight, as long as he is able to read numbers, is given an accurate indication of inclination. This inclination also includes a spirit level components whereby the observer is given both a "coarse" spirit level analog reading and a concurrent "fine" electronic level digital reading.
The inclination gauge disclosed in this copending application includes a bar that is placeable against a surface to determine the extent to which this surface is inclined with respect to the horizontal or vertical axis. Mounted on the bar is a gravity-sensing potentiometer which is connected in a balancing circuit to produce an analog output voltage which attains a null value only when the bar is placed against a vertical or horizontal surface, and which has an amplitude and polarity or phase that depends on the extent and direction of deviation when the surface is sloped.
The analog voltage derived from the balancing circuit is applied to an analog-to-digital converter which output is fed through a display actuator to a digital display to present a reading indicating inclination in terms of angular degree or inches-per-foot. By appropriately setting the conversion factor of the digital converter a reading may be obtained in mils or radians.
Also mounted on the bar are conventional spirit level tubes, one arranged for horizontal and the other for vertical gauging, so that the user of the gauge can observe the displacement of the air bubble in each tube from a null position and thereby obtain an analog reading of the inclination, the analog and digital readings being concurrently presented on the same bar.
The practical advantage of an inclination gauge which affords both an analog and a digital readout is that the user in many situations need not check the "fine" digital reading, where it becomes immediately apparent from the "coarse" analog reading that the surface being gauged is grossly out of line. On the other hand, the fact that the analog and digital readings are concurrently available on the gauge for comparison gives the user an immediate check of any defect in the operation of the gauge.
The potentiometer, which is sensitive to inclination with respect to either the horizontal or vertical plane is formed by a circular cell partially filled with a pool of semi-conductive liquid, and an electrode assembly disposed within said cell and constituted by a disc-shaped center electrode and four electrode segments concentrically-arranged in quadrature relation about the center electrode. The pool partially immerses the center electrode and the lowermost two of said electrode segments when the bar is placed against a surface to be gauged, the distribution of liquid between the immersed electrode segments depending on inclination, the other two electrode segments being connected respectively to the immersed electrode segments.
Because of the quadrature relationship of the electrode segments, regardless of where the bar is placed, a pair of segments will always be immersed; and because of the connection between the two pairs, whichever pair is immersed will be operatively connected to the balancing circuit.
In an actual working embodiment of a gravity-sensing potentiometer of the type disclosed in the copending application, the circular cell includes a pair of opposing walls in parallel relation, and the central disc electrode and the electrode segments surrounding the disc element are fabricated from sheet metal or foil, these elements being secured to one wall of the cell.
Calibration tests conducted with potentiometers of this type indicate a margin of error of about 0.03 degrees. While this error factor in most instances is too small to be of practical significance, when greater precision is required, it is not acceptable.
We have found that this error arises mainly because of surface tension effects which cause the liquid to attain a slightly different level with respect to the electrodes of the assembly than is dictated by the existing inclination. Though one can somewhat reduce surface tension effects by the selection of a liquid having a relatively low surface tension, the liquid chosen for this purpose may lack the proper conductivity and other characteristics desirable in the gravity-sensing potentiometer.
Another factor which comes into play with regard to the accuracy of the gravity-sensing potentiometer is transverse tilt. In this regard, let us first consider a situation in which the inclination bar carrying the potentiometer is placed on a surface to be gauged which is inclined in its longitudinal axis with respect to the horizontal plane, the surface having a transverse axis which extends in the horizontal direction. In this instance, the parallel walls of the potentiometer cell will be positioned vertically, and the surface of the liquid in the cell will be horizontal and at right angles to the cell walls.
Now let us consider a second situation in which the surface to be gauged is again inclined in its longitudinal axis to the same degree as in the first situation, but in the transverse axis is now tilted or inclined with respect to the horizontal direction. In this second case, the parallel walls of the potentiometer will be tilted with respect to the vertical and the horizontal surface of the liquid in the cell will strike a different level on one wall than on the opposing wall.
Since the electrodes which sense inclination are disposed against one of the walls, the resulting reading will contain an error component; for it will not only indicate the inclination of the longitudinal axis of the surface being measured relative to the horizontal plane, but it will also reflect the transverse tilt.